Correct way to remove and add product to order on sales_order_place_after event

Question

I'm trying to use the sales_order_place_after event to remove or add products to the order only after successful payment but facing a few different problems.

There are a few different solutions I could find, but every time I'm there is a different issue:

• Products I'm adding are not correctly deducted from the stock
• Cannot force that newly added product price is 0 when I'm adding more than one. Only the last one got 0.
• The new product must be listed separately on the order/invoice if this product already exists on the order.

I also need to remove some existing products from the order simultaneously and replace them with different ones at the same price.

As this will not affect the order total, so I think that order recalculation is unnecessary.

**** UPDATE ****

Here is one of many ways I was trying to achieve that. It is the closest to the desired result but added qty of product is not reduced from the stock.

public function execute (\Magento\Framework\Event\Observer $observer)
    {

        $order = $observer->getEvent()->getOrder();

        $productId = 123;
        $qty = 2;
        $price = 0;

        $product = $this->productRepository->getById($productId);

        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $orderItem = $objectManager->create('Magento\Sales\Model\Order\Item');


        $orderItem->setProductId($product->getId());
        $orderItem->setSku($product->getSku());
        $orderItem->setQtyOrdered($qty);
        $orderItem->setQty($qty);
        $orderItem->setBasePrice($price);
        $orderItem->setPrice($price);
        $orderItem->setRowTotal($price);
        $orderItem->setOriginalPrice($price);
        $orderItem->setName((string) __('%1 (Promo)', $product->getName()));
        $order->addItem($orderItem);

        try {
            $this->orderResource->save($order);
        }
        catch (\Exception $e) {
            throw new \Magento\Framework\Exception\LocalizedException(
                __($e->getMessage() . " for order {$order->getIncrementId()}")
            );
        }
}

Unfortunately, I'm confused if I should still make those changes on the quote or already edit the order, so I would appreciate it if someone could tell me the correct way to do it, please?